The present invention relates to the field of bags including complementary closure strips designed to enable a user to open and close the bag on successive occasions.
Numerous types of bag and/or closure strip have already been proposed for this purpose.
The present invention applies in particular to bags whose closure strips are opened and closed by means of a slide.
Still more precisely, the present invention relates to bags made from a film plus closure strips which are fed separately to an assembly station where the closure strips are fixed onto the film.
Numerous methods and machines have been proposed for this purpose.
Nevertheless, the person skilled in the art has long been aware that making bags using such means often raises a major difficulty: the thickness of the closure strips brought onto the film makes it difficult to perform heat-sealing across the strips for the purpose of closing and sealing the bags.
Various solutions have been proposed in attempts to overcome that difficulty.
The solution in most widespread use consists in hot pinching the strips together at intervals prior to feeding them to the assembly station. The intervals correspond to the pitch of the bags and the pinches are located in zones that are intended to coincide with the transverse heat-sealing. Such prior pinching also has the advantage of fixing the complementary strips together prior to fixing them to the film.
Such a solution is described, for example, in the following documents: FR-2 778 362, U.S. Pat. Nos. 4,756,622, 5,024,537, EP-0 302 144, U.S. Pat. Nos. 5,092,831, 4,663,915, 5,215,380, 5,046,300, and 4,589,145.
Another technique consists in feeding strips in lengths that correspond to the pitch of the bags instead of feeding the closure strips in continuous form.
That technique is described, for example, in the following documents: FR-2 716 158, FR-2 707 251, and EP-0 528 721.
Nevertheless, that technique has not been widely successful industrially because of its complexity. Specifically it turns out to be very difficult to guide the strips in segment form rather than continuously.
Another solution has been proposed in an attempt to eliminate the drawbacks of all of the means mentioned above. For example, document U.S. Pat. No. 4,876,842 describes an example consisting in detecting any lack of synchronization between the film and the closure strips and in acting on the drive thereof so as to resynchronize the film and the strips.
That technique is promising in theory, but it too has not had the expected industrial development, specifically because of its complexity, and due to relatively poor reliability.
The present invention now has the object of proposing novel means enabling the drawbacks of the prior art to be eliminated, and thus of proposing performance that is better than that in the known prior art.
In the context of the present invention, this object is achieved by a method of manufacturing bags that include complementary closure strips, the method comprising the steps consisting in:
separately feeding a film and closure strips having sequential thin zones at the same pitch as the bags; and
fixing the strips to the film;
the method further comprising the steps consisting in:
detecting loss of synchronization between the closure strips and means acting in the assembly station; and
in the event of loss of synchronization being detected, modifying the size of the sequential thin zones provided in the strips so as to reestablish synchronization.
More precisely, in the context of the invention, the detection means can detect loss of synchronization between the closure strips and the film, and/or loss of synchronization between the closure strips and means that are used for heat-sealing across the closure strips, at least in part, e.g. means for making a transverse line of heat-sealing.
The present invention also provides a machine for implementing the above-specified methods, and bags obtained thereby.